1. Field of the Invention
The present invention relates to an intake air parameter estimating device especially used for estimating a tuning frequency estimating device for an internal combustion engine. More particularly, the present invention relates to a technology for improving the accuracy with which the intake air pressure in the vicinity of an intake valve of an internal combustion engine is estimated.
2. Background Information
The gas inside a cylinder of an internal combustion engine is composed of the residual gas from the previous cycle and the fresh air that is newly inducted into the cylinder. Therefore, in order to know the composition of the gas inside the cylinder, it is necessary to estimate the quantity of the residual gas. The residual gas includes gas remaining in the gap volume of the cylinder and exhaust gas that blows between the exhaust port side and the intake port side during the overlap period when both the intake valve and the exhaust valve are open (hereinafter referred as “blow-by gas”). Therefore, it is necessary to find the quantity of the blow-by gas with good accuracy in order to obtain an accurate estimate of the quantity of the residual gas. The blow-by gas quantity depends on the pressure difference between the exhaust port and the intake port. Therefore, it is necessary to obtain an accurate estimate of the intake air pressure in the vicinity of the intake valve during the overlap period.
Japanese Laid-Open Patent Publication No. 10-153465 discloses a technology for estimating the state of the intake air pressure in view of the backflow caused by blow-by gas. The technology disclosed in the above mentioned publication serves to correct the error in an intake air pressure value detected by an air flow meter resulting from backflow. In order to compensate for the amount of error resulting from the backflow, the intake air temperature is detected and the engine tuning rotational speed is corrected based on the detected intake air temperature. Then the corrected engine tuning rotational speed is used to calculate an correction amount of the air flow rate for correcting the error caused by the effect of intake air pressure pulsation.
More specifically, the above mentioned reference describes a technology whereby the engine rotational speed that resonates with the intake air pressure pulsation (“resonance rotational speed”) is corrected based on the intake air temperature and the amount by which the resonance rotational speed is corrected is used in a correction of the error of the intake air flow rate detected by an air flow meter. In this reference, an intake air flow rate error correction quantity is found based on the throttle opening degree and an engine rotational speed that has been corrected based on a ratio of the intake air temperature and a reference temperature corresponding to a reference state.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved intake air pressure estimating device and tuning frequency estimating device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.